Journal of Fish Biology (1996) 49 (Supplement A), 145-1 56

Preliminary stock assessment of the Patagonian toothfish longline fishery around the Falkland Islands

S. DES CLERS~, C. P. NOLAN*, R. BARANOWSKI AND J. POMPERT* Renewable Resources Assessment Group, Imperial College, 8 Prince's Gardens, London S W7 1 NA, U. K. and * Falkland Islands Fisheries Department, Stanley, Falkland Islands

An experimental fishery for Patagonian toothfish Di,ssosfichus rlcginoides was opened in Falkland Island waters briefly in 1992 and then from April 1994. One to two longlines per vessel were usually deployed at night, mostly fishing for 12-30 h at depths between 600 and 2000 m. The characteristics of the vessel, gear, fishing activities and the data collection and analyses methods are described. An initial evaluation of the authors' current understanding of toothfish biology and population dynamics in Falkland Island waters is given. A first analysis of toothfish numbers caught in 1994 suggests that these are changing at rates faster than expected from simple demographic processes. Therefore, despite an intensive monitoring of catch and fishing effort of each vessel, it is still not possible to derive reliable estimates for the size of the toothfish population currently exploited around the Falkland Islands. Migration patterns in and out of' the fishery need to be understood before a reliable assessment of the fishery can be made. These results, together with current and future lines of research, are discussed in the light of ddtd available from other toothfish fisheries in austral waters.

,V I996 The Fishcrics Socicty 01. tlic Britiali Isles

Key words: toothfish; longline; assessment; Falkland Islands: deep sea

INTRODUCTION

The Patagonian toothfish Dissostichus eleginoides Smitt, 1898 belongs to the family Nototheniidae (' Antarctic cod '), endemic to Antarctic and sub-Antarctic waters. Although not fully known, the life cycle is likely to follow a similar pattern over its amphiaustral distribution, around the Kerguelen Islands in the Indian Ocean, South Georgia, the Falkland Islands, the Southern Argentinean and Chilean edges of the Patagonian shelf in the Atlantic Ocean (Kock et LzZ. , 1985) and the South-east Pacific coast of Chile (Moreno, 1991). Depending on the area, spawning takes place probably between March and August on the continental slope at depths around 500 m. Pelagic eggs hatch to larvae between August and November (Kellerman, 1990), and juveniles probably remain pelagic for a further year, until reaching 15-20 cm in length when they become demersal. Maximum sizes have been estimated to be around 170 cm in Kerguelen, and more than 200 cm in the South Atlantic (Kock et al., 1985), at a maximum age of 20-21 years (Hureau & Ozouf-Costaz, 1980). Preliminary results from scale reading indicate that the oldest toothfish in Falkland waters may be 30 years old. Immature and adult toothfish are mostly piscivorous, with larger sizes inhabiting increasing depths (Duhamel, 1981, 19930,b).

In order to determine an optimal exploitation rate, the Falkland Islands Fisheries Department (FIFD) authorized an experimental fishery to operate briefly in 1992, and continually from April 1994, within specific monitoring

PTel.: +44 171 594 9276: fax: +44 171 589 5319; email: s,desclers('ic.ac.uk

145 0022--1112/96/49A145+ 12 $25.00/0 '(> 1996 The Fisheries Society of the British Isles

146 S . DES CLERS ET AL.

FIG. 1. Chart of the southern Patagonian shelf region showing the Falkland Islands lnterim conservation Zone (FICZ) and the Falkland Islands Outer Conservation Zone (FOCZ). The toothfish longlinc fishery is restrictcd between the 500 and 2000 m depth contours and, in 1994, took place mostly in the south of the FOCZ (shaded area) in Squares A and B. Low tonnages of toothfish were also caught incidentally in the trawler fisheries operating within the FICZ.

requirements for scientific observation and an intensive collection programme for catch and fishing effort data.

Details of fishing operations, data collection programmes and methods of data analysis are presented in the first section. Salient features of the toothfish biology are then detailed, with the authors’ first attempts at estimating the size of the exploited toothfish population. Finally, the discussion is focused on current and future research on the population dynamics of Patagonian toothfish around the Falkland Islands.

MATERIALS AND METHODS

FISHING ACTIVITIES Fishing within 200 nautical miles of the Falkland Islands shoreline has been controlled

since 1987, while it is still unrestricted outside this limit in international waters. Falkland Island waters were divided historically into two zones (Fig. 1). The Falkland Islands lnterim Conservation Zone (FICZ) of 150 nautical miles radius from the geographical centre of the islands was the only zone open to fishing until December 1993. After that, fishing was allowed also in the Falkland Islands Outer Conservation Zone (FOCZ). Longlining for Patagonian toothfish took place sporadically in April, May and Septem- ber 1992, and started in earnest in April 1994, following the opening of the deeper grounds of the FOCZ. Since 1994, a maximum of two longliner vessels has bcen licensed at any given time to target toothfish on grounds deeper than 500 m within the FICZ and FOCZ.

A total of 11 different longlining vessels has taken part in the fishery between 1992 and 1995. These have been relatively small craft (Table I) with around 35 crew, all Chilean owned except for one Bulgarian in 1992, and one South Korean in 1995.

PATAGONIAN TOOTHFISH LONGLINE FISHERY 147

TABLE I. Characteristics of vessels and gear in the toothfish longline fishery in Falkland Island waters between 1992 and 1996

Effort characteristics Range Mean Unit

Vessels Length (overall) Beam Main engine power Daily freezing capacity Gross registered tonnage

Length of longline Number of hooks per line

Gear

39-60 49 m 8.2-10.0 8.8 m 850-2200 1290 bhp

12-30 15 t day-' 380-690 580 GRT

8500-25000 13167 m 5700-12000 8460 hooks

DATA COLLECTION

Since 1987, when fisheries were first regulated in Falkland Island waters, all vessels licensed to fish have reported daily to FIFD the weight of species caught, the effort (time and gear) and position. Also, FIFD has been operating a scientific observer programme to monitor more detailed aspects and collect specimens for on-going research. An observer joins a vessel for a period of 4-5 weeks and works mostly on the factory deck collecting data on species catch composition, measurements and samples of individual fish (e.g. mostly length, weight, sex, maturity and otoliths). An eight-point maturity scale is used by FIFD for all fish species (Table 11).

Given the experimental nature of the toothfish fishery, observer coverage was intensive in 1994, with 130 of the 228 licensed fishing days observed. The local commercial company which charters the longliners also has a representative on each vessel and has been highly supportive of scientific observation activities. In addition to daily radio reports, longliner skippers have to describe fishing operations for each line set in a customized logbook, which was designed after consultation with scientists involved in monitoring other toothfish fisheries under the jurisdiction of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR).

Individual logsheets can describe up to four lines set over 24 h, together with basic environmental observations (sea-surface temperature, sea state and wind speed) being recorded twice per day. For each line set, basic characteristics (length, type of bait, number of hooks set and recovered) are noted, and set patterns (date, time, latitude, longitude and depth) are given for the beginning and end of each operation. Numbers and total weight caught are detailed currently for four species groups (toothfish, skatehay, grenadiers and another which can be specified). Incidental mortality (seabird or marine mammal) is noted and reported daily by radio to FIFD. Finally, the total weight of fish processed daily per species group, and the running cumulative weight of frozen fish on board, are noted on each page.

STOCK ASSESSMENT Since the onset of the experimental fishery in the FOCZ in 1994, there has been too

little information available to develop an age-structured assessment of toothfish catches. For most of 1994, two vessels were fishing mainly in the South of the FOCZ in Squares A and B (Fig. 1). These daily catches have been analysed using a modified DeLury depletion model (Seber, 1982) to estimate the initial population size and mortality rate due to fishing. In the most simplistic depletion model, the exploited population is assumed to be closed both geographically (no migrations) and demographically (no births or deaths), and evenly distributed within the area at all times. In such cases, and provided that fishing removes a large enough portion of the population over time, catch rates

148 S. DES CLBRS ET AL.

TABLE 11. Maturity scale for Dissostichus eleginoides

Name Stage number Description

I

I1

IT1

IV

V

VI

VII

VTTT

Immature

Resting

Early developing

Late developing

Ripe

Running

Spent

Recovering spent

Gonads very small, threadlike, transparent ribbon

Testis translucent. Outer edge becoming pleated. Few situated close to vertebral column.

capillaries present. Ovary small, no eggs distinguishable to the naked eye.

Gonads opaque with network of capillaries evident. Testis heavily frilled, no milt present. A few eggs distinguishable to the naked eye in the ovary.

Gonads occupying two-thirds of body cavity. Testis exude milt when cut. Ovary amber yellow clearly containing eggs distinguishable to the naked eye.

Gonads achieving maximum dimension filling most of body cavity. Testis extremely pleated exuding milt under firm pressure. Ovary containing completely rounded eggs some of which are translucent and ripe.

Milt runs freely from the testis and eggs from the ovary when slight pressure is applied.

Testis and ovary flaccid but not completely empty, capillary network extensive. No opaque eggs left in the ovary.

resorption. Testis and ovary red and empty. A few eggs in state of

should decrease over time with population size. The assumption of total closure can be relaxed, either if migrations or demographic changes cancel out and the population is therefore in a state of dynamic equilibrium, or i T recruitment (or immigration) and mortality other than fishing (or emigration) can be specified. Different variants of DeLury models and associated assumptions were compared informally using the software CEDA (Holden & Bravington, 1992). A modified DeLury model with constant recruit- ment was selected in order to estimate the magnitude of net movement into each grid square. For each vessel in each square, the model is written as follows:

C,=y El N , (1) where: C, is the total catch during week t (numbers of toothfish); E, is the fishing effort during week t (hook-hours); q is a coefficient describing the fishing power or catchability of the vessel (l/hook-hours); and N , is the population size at the beginning of week t (number of toothfish), itself given by the following recursive equation:

~ , = e - M ~ ~ - , - e - ' l . 5 ~ C , - , + R (2) where: M is the constant instantaneous rate of natural mortality; and R is the net number of toothfish that recruited (migrated) into the fishery in the statistical square during the previous week. R is assumed to be constant from week to week over the year.

Thus, the population size at the beginning of week t was computed from the population size at the beginning of the previous week ( N , ,) corrected for the natural mortality incurred over that week (e - Iw), minus the catch (by convention in the middle of the week), corrected for natural mortality during the half week (e-"""), and plus the recruitment R (by convention at the end of the week, hence not subject to mortality). In equation (2), the population size during the previous week ( N , - ,) can itself be expressed as a function of population size, and catch and recruitment of the preceeding week

PATAGONIAN TOOTHFTSH LONGLINE FISHERY I49

( N t P 2 ) , and thus as a function of No, the population size at the beginning of the fishing season.

In CEDA, the model is fitted after a log-transformation of the observed time series of catches in 1994, and provides estimations for the unknown parameter q (catchability coefficient), for No the initial number of toothfish and R the constant weekly recruitment.

RESULTS

CATCH AND EFFORT

In order to minimize potential gear interactions with the various trawling fisheries operating in Falklands waters, longliners have been restricted to fish in waters deeper than 600m. However, demersal trawlers operating on the continental shelf have been catching incidentally around 200 t of young toothfish per year, Maximum toothfish by-catch has been to the south-west of the FICZ on the deeper and colder side of the shelf, and less than 25 t in any one grid square (Fig. 1). In 1994, the total by-catch was 230 t while 2734 t were caught during 385 vessel-days in the longlining fishery. This is presumably because toothfish are at the northern edge of their geographic distribution on the Patagonian shelf.

In April 1992, six longliners were licensed to fish experimentally within the FICZ. Over the 84 vessel-days fished, on average, lines of between 8000 and 11 000 hooks were set at depths between 750 m and 1450 m, with catch rates between 0.6 and 2-9 t vessel-day- '. At the end of May 1992, fishing stopped and commercial operators declared longlining in the FICZ to be uneconomic, probably with reference to other opportunities in the region.

In 1994, after the fishing began in April, longlining was mostly in the FOCZ, and to the south (423 lines set) with minor activity in the north (58 lines set). These lines are indicated in Fig. 1 as short line segments. They were set usually along depth contours between 1000 m and 1500 m, and have been drawn to scale between the latitude and longitude coordinates. Catch rates averaged 7 t per vessel-day, with average incidental ray/skate catches of 0.06 t vessel-day - '. In 1995, catch rates were around 5 t vessel-day-', with 0.2 t of ray. However, as most vessels changed between the 2 years, changes in catch rates cannot be linked directly to changes in toothfish abundance or distribution.

BIOLOGY

In 1992, 1009 Patagonian toothfish were measured and,.in 1994, 8916 were measured and weighed individually (Table 111). The average size of female toothfish caught by longlines was consistently larger than that of males, in all years and areas. Average lengths of both sexes differed between areas. Smaller fish were caught in the FICZ, and larger ones were caught in the Southern FOCZ, although the average depths fished were similar (Table 111). Apart from the size difference, there are no obvious sexual dimorphic characters in toothfish, thus the numbers of males and females sampled during fishing operations provided a good approximation of the sex ratio in the catch for each area. In the FICZ in 1992, smaller males predominated. In the Southern FOCZ, toothfish caught were larger and both sexes were more evenly represented.

150 S . DES CLERS ET AL.

TABLE 111. Total length (cm) and total weight (kg) of toothfish sampled from the longline fishery in Falkland Islands waters in 1992 and 1994

1992

FICZ North FOCZ South FOCZ

Male Female Male Female Male Female

Length (cm) Mean 88.49 96.28 90.13 99.6 99.19 111.28 Min 52 55 57 66 51 58 Max 138 180 135 175 170 225

Mean 8.10 11.98 10.96 16.82 Min I .5 2.5 1.5 1.5 Max 29 65 46 131.6* N examined 623 386 264 179 4224 4249

Weight (kg)

~ ~ ~~

*225-cm long fish could not be wcighed at sea, weight estimated from length-weight function. FICZ, Falkland Islands Interim Conservation zone (average depth fished, 1334 m); FOCZ, Falkland

Islands Outer Conservation zone [average depth fished, 1068 (North) and 1008 (South)].

Observed lengths of toothfish from longline catches in the FICZ in 1992 (a), and the North (b) and South (c) of the FOCZ in 1994 (Fig. 2: low frequencies were not always visible because of large sample sizes, see Table 111) appeared slightly higher than those reported for other longline fisheries. This may have been due mostly to a few larger fish caught in Falkland Island waters where sizes ranged between 50 and 225 cm (Kerguelen: 60-136 cm, Duhamel, 1993b; South Georgia: 50-175 cm, Parkes, pers. conim.; South Sandwich: 50-139 cm, Rubilar et al., unpublished).

The power function between overall weight (kg) and total length (cm) for combined sexes in the Falkland longlining fishery (Table IV) were very close to those from Kerguelen and South Georgia for the sexes combined and separately (Kock et al., 1985). The higher power for females was offset partly by a smaller intercept, and reflected a change of shape in maturing and mature females. It was due mostly to a predominance of females in the few fish > 110 cm (Fig. 2), and resulted in a slightly higher parameter estimate for combined sexes as well. In April and May 1992, the smaller toothfish caught in the FICZ were mostly immature (Stages 1, 2 or 3, Fig. 3). In 1994 in the Southern FOCZ, pre- spawning females (Maturity Stage 4) were caught between April and July, while mature females (Maturity Stages 5 and 6, Fig. 3 ) were caught between August and September. A similar gradual increase in maturity was observed for males, indicating an autumn spawning for the fish caught in the Southern FOCZ in 1994.

STOCK ASSESSMENT The aim of the stock assessment was to estimate the abundance of toothfish at

the beginning of the fishing season, using catch and fishing effort data during that season. In the current absence of age information in the database, it was assumed that the age-structure of toothfish in the population remained stable

PATAGONIAN TOOTHFISH LONGLINE FISHERY 151

2o 7 1 5 1 I

5

n - 50 70 90 110 130 150 170 190 210

Total length (cm) 20

(b)

50 70 90 110 130 150 170 190 210 Total length (cm)

g 10

5

n " 50 70 90 110 130 150 170 190 210

Total length (cm)

FIG. 2. Length-frequency distributions of toothfish caught by longlines in 1992 and 1994 (sample sizes given in Table 111). (a) 1992 Falkland Islands Interim Conservation Zone (FICZ) only. (b) 19941995 North Falkland Islands Outer conservation Zone (FOCZ). (c) 1994-1995 South FOCZ. Solid bars, male; hatched bars, female.

over time. In other words, fishing and natural mortality were assumed to remove identical proportions over all age groups. This assumption was almost certainly incorrect for the very young and very old fish which usually have higher natural

IS2 S . DES CLERS ET A L .

TABLE IV. Least-squares estimates of parameters a and b for toothfish in the Falkland Islands longlining fishery in 1994- 1995, for W=U Lh where L=total length (cm) and W=total weight

w (kg)

Parameter U h r2 P I7

Combined sexes 6.52 l o p 6 3.1045 0.98 <0.001 8916 Male 8.45 l o p 6 3.0448 0.97 <0-001 4488 Female 6.68 l o p 6 3.1023 0.98 <0.001 4428

The regression was performed on log-transformed data

100

80

60

40

20 - 0 Jan Mar May Ju l Sep Nov

100

80

60

40 #

20

0 Jan Mar May Jul Sep Nov

100.- - - - ' 100 -- -

.. .. .

" Jan Mar May Jul Sep Nov Jan Mar May Jul Sep Nov

01 R2 I33 14 @5 B6

FIG. 3. Maturity status of toothfish caught by longlines in 1992 (Falkland Islands Interim Conservation Zone) and 1994 (South Falkland Islands Outer Conservation Zone). Maturity scale is given in Table 11, and sample sizes in Table 111. (a) Malc, FICZ; (b) female, FICZ; (c ) male, South FOCZ: (d) female, South FOCZ.

mortality rates, However, over the period of 1 year examined here, and for the age/size range targeted by the longline fishery, this assumption was considered to be valid. The weekly total number of hook-hours was used as a unit of fishing effort. Between April 1994 and August 1995, 64% (390/797) of all longlines were set in two statistical rectangles (A and B, Fig. l) , each being 0.5" in longitude, and 0.25" in latitude, representing an area of approximately 15 x 17 nautical miles (exact positions of NW corners are 54"15' S-SS"30' W for A and

PATAGONIAN TOOTHFISH LONGLINE FISHERY

0.006

0.005 h e I & 3 0.004

153

- (b) -

-

t" 0.006

0.005

v 2 a, 0.002

0.001

5 e

Week (from 1 January 1994)

0.0°6 0.005

r(

3 3 0.003

a, 0.002

0.001

v

I r

1 1 1 1 1 1 1 1 1 1 1 1 l 1 1 1 1 1 l 1 1 1 l - i I I I I

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Week (from 1 January 1994)

FIG. 4. Time series of mean weekly catch per unit efTort (cpue) for two vessels operating in 1994 in Squares A or B (Fig. 1) of the Southern Falkland Islands Outer Conservation Zone. Discontinuous lines indicate weeks when the vessel either did not fish or fished outside the specified square. (a ) Square A, Vessel 1. (b) Square A, Vessel 2. (c) Square 6, Vessel 2.

54"30' S-55"30' W for B). The time series of catch per unit of effort (cpue) was averaged by week in numbers of toothfish per hook-hour for each of the two vessels fishing in Squares A or B (Fig. 4).

I54 S . DES CLERS E T A L .

TABLE V. Preliminary assessment of the Dissostichus eleginoides longlining fishery in Squares A and B derived from the catch per

unit efl'ort of two different vessels

No Weekly R y ( x l o p 8

Vessel 1 in A 213 936 410 2.157 Vessel 2 in A 182 341 3 50 1.383 Vessel 2 in B 82 897 159 5.253

The fishable area is assumed to be between the 500, and 2000 m isobaths (Fig. l) , natural mortality was fixed at 0.00192 week

No, estimated population size (numbers of toothfish) at the start of April 1994; R, rate of recruitment (numbers per week); q, catchability coefficient (per hook * hours).

.

Three important features were obvious from the cpue time series. Firstly, average weekly catch rates decreased over time until Week 47, for both vessels in the two squares, thus suggesting some local depletion of the exploited popu- lation. Secondly, over the period, the rates of decrease were very different between vessels, and between squares. Starting mid-April (Week 15), the cpue of Vessel I in Square A followed a steady decline from about 0-004 to 0.002 toothfish hook-hour - ' over 30 weeks. The rate of decline for Vessel 2 in Square A was similar, although starting from a lower value. Catch rates for Vessel 2 in Square B were more variable, decreasing faster from a higher initial value between Weeks 20 and 35, and then remaining roughly constant until Week 47. Thirdly, Vessel 2 catch rates increased in both squares at the beginning of 1995 (Week 53), apparently contradicting the authors' assumption of a closed population in the DeLury model.

For the assessment, the constant weekly mortality rate M was taken to be 0.00192 (or 0.1 year- '), in line with values used for toothfish by CCAMLR (Moreno & Rubilar, 1992) for the Patagonian shelf. Estimates for the unknown parameters No, R, and q, were derived from the best fit between observed and predicted weekly catches given by the catch equation (l), using a least-squares criterion after log-transformation (Holden & Bravington, 1992). A different set of parameters was estimated independently for each vessel in each of the two grid squares (A and B), over the time period when catch rates were decreasing (Weeks 15-50) in the Southern FOCZ in 1994 (Fig. 4; Table V).

DISCUSSION

The authors' first attempts at modelling the population dynamics of toothfish around the Falkland Islands have provided encouraging results. However, these preliminary results cannot be generalized. Estimation of parameters for grid Squares A and B cannot be extrapolated to other grid squares for several reasons. First, patterns of cpue depletion are very different between vessels (Fig. 4). Thus, estimations can be compared at first only between areas fished by the same vessel. In Table V, Vessels 1 and 2 give similar results for Area A. Results from Vessel 2, however, suggest an important difference between Areas A and B.

PATAGONIAN TOOTHFISH LONGLINE FISHERY 155

These differences, in both initial estimated population sizes and weekly recruit- ment, can be explained in part by the topography of the two areas. Square A is totally within the fishable domain between isobaths 500 m and 2000 m. Square B is in a steeper area, and the longlines have been set in a smaller surface area. This may explain also the difference in estimated catchability for Vessel 2 between the two areas (Table V). The higher coefficient for B may indicate a local density of toothfish in Area B greater than in Area A.

Most importantly, the model’s underlying assumption of limited and constant recruitment into the fishery did not hold after a period of a few weeks. In the two time series longer than 30 weeks, catch rates increased dramatically after the period of steady decline which was used in the assessment (Fig. 4). Depletion models are simple but crude tools. Their use for the assessment of toothfish fisheries has proved increasingly limited (Parkes et al., 1995), as both the demography of such a long-lived species and the likely extensive migrations of toot hfish become better known.

Additional data are being sought in order to develop a more realistic assessment model. Several projects on the ageing of toothfish have been initiated recently by FIFD, and otoliths, scales and vertebrae are now collected regularly. These data make it possible to analyse the presence of larger fish in Falkland Island waters compared with the other longline fisheries in the region, in terms of differences in growth. It may be possible that larger fish are caught in Falkland Island waters, even when similar depths are fished around South Georgia (Agnew & Moreno, 1992), or Kerguelen (Duhamel, 1993a,b) as growth would be enhanced in the proximity of the Patagonian continental shelf. However, differences in sizes caught may simply reflect different catchabilities between fisheries (hook types and sizes, Agnew & Moreno, 1992). Finally, differences in exploitation rates could explain also the presence of larger fish in Falkland Island waters where fishing pressure has been kept at a low level.

A better understanding of toothfish migrations is needed to provide a sound basis for the fishery’s management. Several research programmes have been put forward recently to identify population units and movements by different methods. Toothfish caught in demersal trawl research cruises have been tagged in 1995 and 1996. A tagging programme using individually marked circular (tuna) hooks tied to detachable snoods is being evaluated. A study of the parasite fauna of D. eleginoides is also underway and may identify parasite species which can be used as biological tags for stock discrimination purposes. Morphometric evidence has shown that D. eleginoides populations around the Falkland Islands and around South Georgia are distinct from each other (Kock et al., 1985). However, it is likely that the mainly adult toothfish population in Falkland Island waters is replenished regularly from nursery areas in sub-Antarctic waters. Similarly, sudden changes in catch rates on a weekly basis suggest that the home range of adult toothfish is far greater than that of the longliners’ fishable area around the Islands, between the 600 and 2000 m depth contours.

Research on the Falkland Islands Fisheries at Imperial College is funded through a multi-annual programme by the Falkland Islands Government, Fisheries Department, Stanley. The data presented have been collected on board longliners by dedicated observers. often in uncomfortable conditions. The contributions of scientific observers

I56 S. DES CLERS ET AL.

Adam Cockwell, Tom Blasdale, Martin Cox and Alberto Criado are gratefully acknowl- edged. An earlier version of the manuscript was grcatly improved thanks to a referee’s detailed and helpful suggestions.

References

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Moreno, C. A. (1991). Hook selectivity in the longline fishery of Dissostichus eleginoides (Nototheniidae) off the Chilean Coast. Selected ScientiJic Papers SC-CCAMLR- SSP18, 107-1 19.

Moreno, C. A. & Rubilar, P. S. (1992). Notas sobre la mortalidad natural de Dissostichus eleginoides de la subarea 48.3. Selected Scientific Papers SC-

Parkes, G., Moreno, C., Pilling, G. & Young, Z . (1995). The use of stock depletion models for the assessment of local abundance of Toothfish Dissostichus eleginoides. Selected Scientijic Pupers SC-CCAMLR-SSP11 2.

Seber, G. A. F. (1982). The Estimation of A n i n d Abundance and Related Parameters, 2nd edn. London: Charles Griffin.

CCAMLR-SSP19, 21-30.